The commonly used methods to model grid-connected VSC systems to study the oscillatory phenomena are the state-space and impedance-based modelling approaches [1]}, [2]}. The first one represents the system as a set of linear equations in the time domain, but it requires detailed information of the control code which is possibly not available. On the other hand, impedance-based modelling approach is based on the impedance characterization of the system (e.g., detailed knowledge about the converts is not needed) which can be expressed as a transfer function in the s-domain. Stability criteria such as the Nyquist stability criterion [3]}, [4]}, impedance-based stability criterion [5]}, [6]}, [7]}, and the positive-net damping stability criterion (PND) [8]}, [9]} have been used to study the controller influence of a single VSC over the stability of simple networks in the frequency domain.
